UCMR 3 Low-Level VOC Analysis by Purge and Trap Concentration and GC/MS using Selective Ion Monitoring

Importance of the Topic

Monitoring unregulated contaminants in drinking water is essential to protect public health. Volatile organic compounds (VOCs) at part-per-trillion levels pose a challenge for analytical laboratories. The U.S. EPA’s Unregulated Contaminant Monitoring Rule 3 (UCMR 3) demands sensitive, reliable methods to detect trace VOCs in water.

Objectives and Study Overview

This study aims to validate a low-level VOC analysis workflow in accordance with UCMR 3. Key goals include:

• Establishing calibration curves over the 2.5–1000 ppt range for nine target VOCs.
• Determining method detection limits (MDLs) at sub-ppt to low-ppt levels.
• Demonstrating repeatability of internal standards and surrogates.

Instrumentation Used

An integrated system was configured as follows:

• Sample concentration: Teledyne Tekmar Stratum Purge and Trap Concentrator (PTC) with proprietary #9 analytical trap.
• Autosampler: AQUATek 100 for automated sample introduction.
• Gas chromatograph–mass spectrometer: Thermo Trace GC Ultra coupled to Thermo ISQ single quadrupole MS.
• Column: Restek Rtx-624 capillary (20 m × 0.18 mm × 1.0 µm).
• Carrier gas: Helium at 1.0 mL/min, split ratio 25:1.
• Detection mode: Single ion monitoring (SIM) to achieve ppt-level sensitivity.

Methodology and Instrumentation

Sample preparation involved a 1 ppm methanolic stock of the nine VOCs. An eight-point calibration (2.5–1000 ppt) was prepared in deionized water. Internal standards and surrogates were added at 50 ppt to each sample. GC oven programming ramped from 45 °C to 240 °C, and the MS was operated in EI+ mode with selected ions for quantification and confirmation. Method detection limits were calculated from seven replicate analyses at 10 ppt using the EPA’s statistical approach (t=3.143 for n=7, 99% confidence).

Main Results and Discussion

Calibration performance was excellent, with quadratic regression coefficients (r2) ≥ 0.9994 for all nine VOCs. Internal standard reproducibility showed %RSD values below 6%, confirming method precision. MDLs ranged from 0.59 ppt (sec-butylbenzene) to 2.16 ppt (1,3‐butadiene), all well under the UCMR 3 requirement of 2.5 ppt. Overlayed SIM traces demonstrated clear separation of quantification and confirmation ions without interferences. These results confirm the method’s suitability for trace VOC monitoring in compliance with UCMR 3.

Benefits and Practical Applications

The validated purge and trap–GC/MS approach offers:

• Sub-ppt detection capability for critical VOCs in drinking water.
• High throughput via automated sample handling.
• Robust quantitative performance meeting regulatory requirements.
• Applicability for routine compliance and research laboratories.

Future Trends and Opportunities

Emerging directions include coupling purge and trap with high-resolution MS for enhanced selectivity, integrating predictive analytics for real-time contaminant screening, and expanding analyte scope to new unregulated compounds. Further automation and remote monitoring could streamline large-scale water quality surveillance.

Conclusion

The combined Teledyne Tekmar Stratum PTC, AQUATek 100, and Thermo ISQ GC/MS system delivers robust, sensitive analysis of UCMR 3 VOCs at part-per-trillion levels. Calibration linearity, low MDLs, and reproducible internal standards confirm its readiness for regulatory and research applications in water quality monitoring.

References

• U.S. Environmental Protection Agency. Unregulated Contaminant Monitoring Program. UCMR 3.
• U.S. Environmental Protection Agency. Method 524.3: Measurement of Purgeable Organic Compounds by Capillary Column Gas Chromatography/Mass Spectrometry, Revision 1, 2009.